The electric powered passenger vehicle has long been considered one of the most attractive alternatives to conventional internal combustion engine driven types from the standpoint of overall efficiency, environmental impact and, most recently, alternative fuel capability. Many commercial enterprises and private individuals, some under the auspices of the Federal Government, have proposed various approaches to implementing an electrically powered vehicle. To date, there have been virtually no commercially successful vehicles produced on a large scale. A large number of approaches to the implementation and control of an electric vehicle are evidenced in the patent literature. Most of the approaches fall within one of three general categories of motive power source. These categories are hybrids, D.C. motor drives and induction motor drives. The first type, that most frequently found in the patent literature, is the hybrid vehicle, comprising a small gasoline fueled internal combustion engine which mechanically drives an electrical generator which, in turn, supplies electrical energy to an A.C. or D.C. motor. With this arrangement, the gasoline engine can operate at a constant speed (at a relatively high efficiency) and achieve a substantial fuel saving compared with an engine experiencing the conventional wide range of operation. A shortcoming of many hybrids is that they are relatively heavy, requiring an electrical generator and motor as well as the gasoline engine. Additionally, the engine requires substantial amounts of volatile liquid fuel and generates exhaust emissions.
A second approach taken in the development of electric vehicles is the use of a bank of batteries which supply electrical energy to a D.C. motor. A variable speed motor drive circuit provides easy and versatle control of a vehicle. The principle advantage of this arrangement is that a D.C. motor control system requires a relatively simple power and control circuit. Unfortunately, this advantage is often more than offset by the relatively large initial cost and maintenance expenses of the motor itself. In addition, D.C. machinery is relatively heavy and bulky, factors which do not lend themselves well to implementation within a lightweight compact vehicle. Finally, D.C. motors inherently require choppers and commutators which create sparks and RF polution which can be controlled only at additional expense.
The third, and most attractive approach from the applicant's viewpoint, is a vehicle employing a battery bank and an A.C. motor. A.C. motors are relatively lightweight, inexpensive and efficient when compared to D.C. motors. A.C. motors, with no brushes or commutators, are more rugged and reliable then their D.C. counterparts and require substantially less maintenance. Related to the power-to-weight ratio is the fact that A.C. machines can be driven at substantially greater speeds than D.C. motors. Because A.C. motors do not generate sparks, they can readily be employed in dusty, explosive and highly humid atmospheres or high altitudes. Additionally, A.C. motors can be liquid cooled if the application so requires. Although typically superior to D.C. motors in electric vehicle applications, A.C. motors often require complex control circuits which are dedicated to associated vehicle drivetrains and can be extremely bulky and expensive. To date, virtually all A.C. electric vehicles have employed multi (usually three) phase design strategies. Although three-phase machinery. has many advantages as set forth hereinabove, three-phase inverter costs and complexity have proven to be extremely high. In relatively large load applications, such as that required in a passenger vehicle, appropriately sized solid state switching devices such as SCR's or transistors are often extremely expensive. In addition, three-phase inverters, by their nature, dictate a multiplicity of components, including switching devices, again increasing system cost.
Single-phase permanent magnet motors have not been widely accepted for traction drive applications because they share the characteristic of all single-phase AC motors in that no starting torque may be available. A related problem is found in such machines ability to start in either direction, requiring the provision of expensive directional governors. To ensure startup, many electrical and mechanical approaches, such as pole shaving and kick-starting, have been suggested. With pole shaving, each pole is ground so that the air gap increases circumferentially across the pole face to effectively shift the reluctance torque zero point from the electrical torque zero point. A motor with shaved holes can, however, still fail to start such as when the rotor is positioned where the electrical torque is zero due to the slope that the vehicle is parked upon. With kick-starting, an actuator mechanically rotates the rotor a few degrees at the same time electrical power is applied. This method has problems, because logic must be provided to sense which direction the motor should be pushed. Although effective for limited applications, these methods have proven to be unduly expensive or limited to small, fractional horsepower motor application.
It will be apparent from the reading of the specification that the present invention may be advantageously utilized in many different traction drive applications, especially land vehicles. However, the invention is especially useful when applied to electric powered passenger commuter vehicles, and will be decribed in connection therewith.